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Full text of "Handbook Of Chemical Engineering - I"

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394
CHEMICAL ENGINEERING
is considerably higher than trough or rotary dryers, as the material is dried on the drum in a very thin film.
Drying Under Vacuum.—The most common type is the standard shelf dryer arranged very similarly to the apparatus shown in Fig. 31. However, the housings have to be built substantially in order to withstand the outside pressure. The vapors are condensed in either surface or barometric condensers. This type
FIG. 36,
FIGS. 36, 36A.-
FIG. 36A. -Atmospheric drum dryers.
is expensive in operation, as it requires much labor for the charging and discharging of the material in the trays placed on the shelves of the dryers. The rotary dryer shown in Fig. 33 is also used frequently under vacuum, and has given excellent results. A special type of apparatus is the ribbon dryer specified in Fig. 37, which really is a continuous shelf dryer. The liquid is spread evenly over the surface of the ribbon by special feed rollers, or spraying under high pressure. The ribbon itself consists of a fine mesh wire screen treated with a gelatinous coating, so that the liquid will never come in contact with the